Optical metrology is often utilized to measure optical and/or structural characteristics of either device or test features during semiconductor manufacture. For example, optical or structural characteristics may include critical dimensions such as height, side wall angle, pitch, linewidth, film thickness, refractive indices, and overlay between different layers or between exposures within a single layer. Apodization may be implemented in optical metrology systems to control angular and spatial distribution of illumination at well-defined locations along the optical path. Apodization is particularly important when metrology accuracy and precision depends on the ability to retrieve high fidelity spectroscopic or angular information from small metrology targets. In such cases, there is a need to prevent signal contamination resulting from either unwanted scattering from areas outside a designated metrology target on a sample or scattering from intermediate optical components or apertures along the optical path.
In the case of an angle resolved (pupil imaging) scatterometer, a known practice in the art is the combination of: (1) a simple flat top aperture (pupil) stop in the illumination path which restricts the illumination numerical aperture (NA) so that different diffraction orders from the metrology target can be isolated in the collection pupil; and (2) a simple flat top field stop in the illumination path to localize illumination on a small target. With the foregoing architecture, the illumination field stop becomes the limiting aperture of the pupil imaging system. The hard edges of the illumination field stop cause ringing in the images of the illumination aperture stop, and the ringing results in interaction or interference between orders (e.g. 0th order and 1st order) in the pupil image. One method of resolving this problem is field apodization in the illumination path of the optical metrology system. With field apodization, the introduction of a smoothly varying transmission function in the field results in a smoothly varying and rapidly decaying function in the conjugate pupil plane, effectively suppressing the ringing which results in interference between orders.
The foregoing approach may be appropriate for a spatially incoherent system with illumination etendue to spare. However, for optical metrology systems employing coherent illumination sources, such as a laser-based system with high spatial and temporal coherence, substantial noise issues emerge. Shaping the spatially coherent illumination beam so that it has low tails in the field plane is desirable in order to minimize periphery contamination and diffraction by the edge of the target. Low tails in the pupil plane are desirable to minimize interaction and interference between diffraction orders and clipping by the objective pupil. The beam can potentially be shaped by some combination amplitude and phase apodization in the illumination field stop or the illumination aperture stop. To average out the effects of target noise, it is desirable to scan the spatially coherent illumination spot over the target during a measurement. If beam shaping is performed at the illumination field plane, and the spot scanning mechanism is situated before this field plane, then the beam shape in the field and pupil planes will change as the spot scans across the field apodizer. This introduces fluctuations in the overall beam intensity as well as asymmetries in the distribution of light in the pupil.